To the Editor.—The report of Brunell et al1 is comforting because more than 98% of children had humoral antibody titers against measles, mumps, and rubella (MMR) when tested two to 19 months after immunization. This proportion of seropositive children is higher than we reported in a study of immunized Minneapolis children.2 However, a number of differences between our study and the one by Brunell et al invalidate direct comparisons. For example, the period from immunization to testing of our subjects was nearly twice as long as the Texas study, which also did not include children immunized before 15 months of age. The most important difference between the two studies is that the vaccines were not the same. In January 1979, the rubella component of MMR vaccine was changed; RA27/3 strain replaced the HPV-77 DE-5 strain. RA27/3, the rubella antigen given to the Texas children, is more immunogenic